1. Field of the Invention
The present invention relates to a system for controlling a gear ratio of a continuously variable transmission and, more particularly, to a system for performing a speed change control when a driver demands an abrupt acceleration.
2. Related Art
The speed change control in the continuously variable transmission is executed by determining a target input speed on the basis of running conditions such as a demand for a driving force, e.g., an accelerator opening (or depression), or the vehicle speed or on the basis of a manual selection by the driver, and by controlling the gear ratio so that the actual input speed may be equal to the target input speed. For this speed change control, the intended target input speed is finally achieved not by changing the gear ratio so that the actual input speed may be instantly equal to its target value, but by setting a transient target input speed determined from the target input value, by feedback-controlling the gear ratio of the continuously variable transmission so that the actual input speed may be equal to the transient target input speed, and by updating the transient target input speed sequentially. As a result, the speed changing rate is determined on how to set the transient target input speed. Therefore, the ordinary speed change is executed by setting the transient target input speed delayed by first-order time lag from the target input speed so that it is executed at such a rate as to raise neither any shock or the delay feel of the speed change.
With this setting of the speed changing rate at all times, however, a demanded acceleration feel may not be obtained due to the delay in the speed change when an abrupt acceleration is demanded by depressing the accelerator pedal abruptly and deeply. In order to eliminate this disadvantage, in the invention disclosed in Japanese Patent Application Laid-Open No. 63-68426, a downshift of a high speed changing rate is executed by changing the target input speed of the continuously variable transmission stepwise on the basis of the decision of an abrupt acceleration, if this abrupt acceleration is manually effected by the driver in a vehicle having the continuously variable transmission mounted thereon. When the gear ratio of the continuously variable transmission is to be feedback-controlled, more specifically, the amount of control is increased the more for the larger deviation between the present value and the target value so that the speed changing rate becomes the higher. Therefore, the increase amount of the transient target input speed is enlarged or changed stepwise so that the speed changing rate is raised to improve the acceleration response to the abrupt acceleration demand.
In the prior art, the abrupt speed change described above for increasing the target input speed stepwise is executed when the decision of the abrupt acceleration is held by the operation of the drive for the abrupt acceleration, as effected by the abrupt and deep depression of the accelerator pedal by the driver. Therefore, a serious longitudinal vibration (or a surging) or a resulting shock may occur when an abrupt speed change is executed in the driven state where a decelerating manipulation is made by returning the accelerator pedal to drive the engine forcibly by the running inertial force.
When the abrupt acceleration is manipulated for the engine in the driven state, more specifically, the output shaft torque changes from the state of a negative torque to the state of a positive torque so that the looseness of the drive line is reduced to reverse the twist of the elastic line. As a result, there occurs the so-called xe2x80x9clongitudinal vibration (or the surging)xe2x80x9d to cause the pitching or rocking of the vehicle body in a back and forth direction. In order to prevent this pitching or rocking, the so-called xe2x80x9crounding controlxe2x80x9d for raising the engine torque gently is executed for the abrupt acceleration in the driven state.
On the other hand, the gear ratio control of the continuously variable transmission, accompanying the manipulation for the abrupt acceleration, is performed by raising the transient target input speed stepwise to raise the gear ratio abruptly. With the gear ratio being thus controlled, the inertial force accompanying the rise in the gear ratio acts as a negative torque on the engine.
When the rounding control of the engine torque and the abrupt speed change control of the continuously variable transmission progress simultaneously as the decision of the abrupt acceleration is held, these two controls interfere with each other to delay the rise in the engine torque. As a result, the engine is still in the driven state at the instant when the rounding control of the engine torque is ended, so that the engine torque is abruptly raised from that state. After all, even if the rounding control is executed for raising the engine torque, the changing rate (or the changing gradient) of the torque is raised at the instant when the vehicle changes from the driven state to the drive state, that is, when the output shaft torque of the engine is switched from the negative torque to the positive torque, thereby to cause the serious surging and the resulting shock.
A main object of the invention is to provide a speed change control system which can prevent the surging or the shock even if an abrupt acceleration is manipulated when a prime mover is in the driven state, without deteriorating the acceleration responsibility.
Therefore, the speed change control system of the invention is characterized in that the abrupt speed change control of the continuously variable transmission is executed after the vehicle was changed from the driven state to the drive state. According to an aspect of the invention, more specifically, there is provided a controller for effecting an abrupt speed change by judging an abrupt acceleration demand of the driver, when it is judged that the driver demands the abrupt acceleration. Moreover, the controller comprises: drive state deciding means or a judging device for judging, when said abrupt acceleration demand is one from the driven state of the vehicle, that the vehicle has changed from the driven state to the drive state; and speed change control means or a shift controller for controlling said continuously variable transmission to effect said abrupt speed change, after it is judged that the vehicle has changed from the driven state to the drive state.
In this invention, therefore, if the vehicle is in the driven state when the abrupt acceleration is demanded, it is decided by the drive state deciding means that the vehicle has changed from the driven state to the drive state. If it is decided that the vehicle has changed from the driven state to the drive state, moreover, the abrupt speed change control is executed in the continuously variable transmission. Even if the rounding control of the output torque of the prime mover is executed in response to the abrupt acceleration demand, therefore, the interference between the two controls and the resulting delay in the change to the drive state are avoided because the abrupt speed change control of the continuously variable transmission is not executed yet at that instant of execution. Moreover, the abrupt speed change control of the continuously variable transmission is executed after the change of the vehicle into the drive state so that the changing rate (or the changing gradient) of the torque at the switching time from the driven state to the drive state is relaxed to prevent or suppress the surging and the resulting shock.
Said speed change control means or the shift controller in the invention may be constructed to control the continuously variable transmission to interrupt the speed change in the continuously variable transmission, if said abrupt acceleration demand is one from the driven state of the vehicle, for the time period from the decision of said abrupt acceleration demand to the change of the vehicle to the drive state.
With this construction, the speed change control of the continuously variable transmission is not executed after the decision of the abrupt acceleration demand was held and before the change from the driven state to the drive state. In this time period, therefore, it is possible to execute the control the prime mover on the basis of the abrupt acceleration demand, such as the rounding control of the output torque solely. As a result, the prime mover can be controlled easily and precisely, and the timing for starting the abrupt speed change control in the continuously variable transmission can be properly set.
Moreover, said speed change control means or the shift controller in the invention may be constructed to control the continuously variable transmission to perform the abrupt speed change in the continuously variable transmission, if said abrupt acceleration demand is one from the drive state of the vehicle, when said abrupt acceleration demand is decided.
With this construction, the abrupt speed change control of the continuously variable transmission is executed instantly in response to the abrupt acceleration demand, if any in the drive state, so that the shock is not deteriorated to improve the acceleration responsibility.
Moreover, said speed change control means or the shift controller in the invention may be constructed to control the gear ratio of the continuously variable transmission so that an input speed may be a target input speed set on the basis of the running state of the vehicle, and to make an increase amount of the target input speed larger than that at a speed change other than the abrupt speed change when the abrupt speed change is to be performed.
With this construction, in the case of an abrupt speed change, the target input speed is drastically raised to raise the speed changing rate so that a speed change according to a demand can be made.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read with reference to the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustrations only and are not intended as a definition of the limits of the invention.